* Setting `script.painless.regex.enabled` has a new option,
`use-factor`, the default. This defaults to using regular
expressions but limiting the complexity of the regular
expressions.
In addition to `use-factor`, the setting can be `true`, as
before, which enables regular expressions without limiting them.
`false` totally disables regular expressions, which was the
old default.
* New setting `script.painless.regex.limit-factor`. This limits
regular expression complexity by limiting the number characters
a regular expression can consider based on input length.
The default is `6`, so a regular expression can consider
`6` * input length number of characters. With input
`foobarbaz` (length `9`), for example, the regular expression
can consider `54` (`6 * 9`) characters.
This reduces the impact of exponential backtracking in Java's
regular expression engine.
* add `@inject_constant` annotation to whitelist.
This annotation signals that a compiler settings will
be injected at the beginning of a whitelisted method.
The format is `argnum=settingname`:
`1=foo_setting 2=bar_setting`.
Argument numbers must start at one and must be sequential.
* Augment
`Pattern.split(CharSequence)`
`Pattern.split(CharSequence, int)`,
`Pattern.splitAsStream(CharSequence)`
`Pattern.matcher(CharSequence)`
to take the value of `script.painless.regex.limit-factor` as a
an injected parameter, limiting as explained above when this
setting is in use.
Fixes: #49873
Backport of: 93f29a4
791a9d5102